The invention concerns a method and equipment for mixing hard granules into tyre tread bodies and furthermore into sundry rubber strips and tread stock, whereby rubber stock is extruded into tread bands of a suitable shape which are rolled under control, in a suitable thickness and number of layers, in a circular process over a pre-treated tyre or other material (object) which is subsequently patterned and vulcanised.
Vehicle tyres consist, broadly speaking, of a fibre-reinforced carcass (bound with wire or rope) which forms the inner layer of the tyre, and an outer wearing layer which is made of vulcanised rubber or comparable material and is profiled into a pattern over the reinforced tyre carcass.
Tyres are manufactured according to the above specification but retreading of them is also a familiar process. When the pattern of the wearing surface (the tread) has worn down so far that is ceases to serve its function, the remainder of the wearing surface is buffed and a new tread is placed over the fibre-reinforced carcass. One way in which this is done is by overlaying the circular process of the tyre carcass with an extruded rubber tread band of a suitable thickness, which is then profiled into a new pattern and vulcanised.
The role of the outer layer of the tyre is to form a resistant and wearing surface against the surface on which the vehicle is driven. Many methods are known for increasing tyre resistance and reducing skidding of vehicles on a slippery surface such as ice or snow. Particular mention should be made of spikes which are driven into the tyres after conventional manufacture, and snow chains.
In recent years and decades, ideas have been proposed for mixing and/or anchoring of various types of material into or onto the wearing surface of tyres.
Methods are known for mixing hard granules into the tread body of a tyre in order to increase its resistance on a slippery surface, in particular snow and ice. The U.S. Pat. No. 2,672,910 from 1948 describes the production of a tyre in which coated hard granules are mixed with the raw rubber which is used for production of the tyre. By coating the granules improved bondage is obtained with the rubber body.
Furthermore, methods have been developed for re-treading. A worn tyre is pre-treated (buffed) to prepare the rolling of a new tread body onto it. A new layer of rubber is wrapped around the circular process of the tyre and then profiled into a pattern, cf. German Patent DE 4.025459 A1, which concerns general re-treading of used tyres.
European Patent Applications EP 0 454.025 A1 and EP 442.155 A1 furthermore describe methods for production of tyres and/or re-treading, whereby coated hard granules are mixed into the raw rubber before extrusion and rolling on the pre-treated tyre.
The above patents describe methods and equipment for production of treads and tyres. They describe the mixing of hard granules into the tread body of the tyres in order to increase the tyre""s resistance against the driving surface and to reduce vehicle sliding on ice or snow.
None of the aforementioned methods or equipment for mixing hard granules has achieved widespread acceptance, as a result of shortcomings of these methods in the production process.
The disadvantage of these methods has been the cost involved in mixing the hard granules into the rubber body before extrusion, for reasons including substantial wear and tear to the equipment.
The required amount of hard granules must be considered excessive if they are mixed with the entire raw material body. Nor has it proved possible to control the final location of the hard granules in the wearing surface of the tyre, which must also be considered a disadvantage.
Yet another disadvantage is the inadequate bondage yielded between the fibre-reinforced carcass and the granulated tread body, since the granules show a tendency to impair the bondage between the carcass and the wearing surface. Furthermore, mixing granules into the entire rubber body is considered to reduce the flexibility of the tyre carcass.
The aim of the invention is to present a method and equipment which are at once straightforward in use and production, and economical to operate, and which also enable control to be maintained over the mixing and location of hard granules in the wearing surface of tyres. One of the major advantages of the method for which the patent is being applied is that it does not require extra labour for re-treading of tyres.
By manufacturing tyres and other treads with a mixture of hard granules, the following aims are achieved:
Increased vehicle resistance to slippery surfaces.
Reduced road wear compared with the use of spiked tyres.
Enhanced general driving properties compared with the use of spiked tyres or snow chains.
These properties have been confirmed in tests by VTI of Sweden (Swedish Public Roads Administration) and BAST of Germany (German Public Roads Administration or Bundesanstalt fxc3xcr Stassenwesen).
These aims are achieved by the invention""s distinctive feature of distributing the hard granules according to a predetermined configuration onto at least part of the surface of the extruded rubber tread band before the next layer is rolled in a circular process onto the pre-treated tyre, using dedicated equipment so that the granules will be mixed into the patterned wearing surface of the tyre after profiling and vulcanisation.
A further distinctive feature of the invention is that the hard granules are distributed according to a predetermined configuration onto at least part of the surface of the extruded rubber tread band, thereby allowing the extent to which the tread band is covered with hard granules to be controlled, and likewise the distribution of the granules into predetermined parts of the patterned and vulcanised tyre.
A further distinctive feature of the invention is that 0.01 to 0.1 gm of hard granules are distributed onto each square centimeter of rubber tread band which is covered with hard granules, with each granule of the diameter range 1-3 mm, particularly 1.5-2.0 mm.
Furthermore, a distinctive feature of the invention is that the hard granules are distributed according to a predetermined configuration onto the part of the surface of the extruded rubber tread band which, after rolling onto the pre-treated tyre carcass surface, forms the outer surface of the vulcanised and patterned tyre, and that the granule distribution equipment is located after the extruder in the production process and above the extruded rubber tread band, and in front of the tyre, and that the granules are distributed onto the rubber tread band before it is rolled on to the tyre.
The equipment is unique insofar as it consists of a funnel into which the hard granules are fed before distribution, and a feeder (revolving rubber-clad cylinder which feeds a suitable quantity of granules from the funnel onto a track for controlling their delivery onto the rubber tread band, and the distribution device (the track) can be adjusted to control the direction and speed at which the granules are delivered onto the extruded rubber tread band and match as closely as possible the direction and speed of the band itself, and the quantity of granules fed and distributed from the funnel is controlled by a feeder door and by the speed of the cylinder, thereby controlling that the granules are located on the tread band at a suitable density.
Much research has been conducted into the properties of tyres which are produced using the new method described above. Very thorough testing by BAST of Germany has convincingly demonstrated that road wear caused by tyres with mixed hard granules is greater than that caused by xe2x80x9cwinter tyresxe2x80x9d but considerably less than the average road wear caused by 9 types of spiked tyre. Tests at BAST revealed that, setting road wear caused by nailed tyres at a reference point of 100%, wear caused by the above-mentioned hard-granule tyres measures 7% and that caused by winter tyres 2%.
Tests conducted by the Swedish Public Roads Administration of Linkoping show that hard-granule tyres clearly displayed much better road-holding properties under slippery conditions than the same type of tyre without hard granules. The tests also showed that hard-granule tyres generally display better road-holding properties than spiked tyres when rolling along the road surface. This is a particular advantage, for example, on bends and where anti-braking systems prevent the wheels from locking completely.
In all likelihood, many types of hard granules may be used for mixing. The trials which have been made have used granules of SiC (silicon carbide, xe2x80x9ccarborundumxe2x80x9d) and Al2O3 (aluminium oxide), which have been coated with familiar bonding materials and/or adhesives.